A bar code wand reads a bar code symbol over which it is drawn by illuminating the symbol with a beam of light and detecting the amount of the illuminating light that is reflected from the symbol. In essence, a bar code symbol generally consists of an alternating series of reflective and nonreflective bars arrayed in a linear pattern. The variations in reflective and nonreflective bars determines the amount of reflected light that is detected as the illuminating light scans the bar code symbol. The detected light can be converted into an electrical signal which is processed to determine the information contained in the symbol. The variations in the amount of reflected light contain information in the form of variations in duration of the electrical signals produced when the bar code wand is drawn across the bar code symbol.
Conventionally, the bar code symbol is illuminated by a single beam of light projected through a lens and the reflected light is received along the same optical path. The illumination beam is generated by a light source such as a laser diode, which is then shaped to a desired form by passage through one or more apertures and associated lenses for focusing the beam at a focal point having a desired range. It is recognized that the focus of the beam degrades as the wand is moved so that the bar code symbol being scanned is no longer precisely at the focal point. However, there is a range of distances, called the "depth of field," within which the bar code wand can be moved and still have the illumination beam sufficiently close that the bar code symbol can be accurately read. The depth of field is centered about the focal point; therefore, a wand can be used to read bar code symbols which are at distances other than exactly at the focal point.
The performance of a bar code wand is affected by the optical path along which the reflected light travels. Of significant importance is the amount of the reflected light received by the lens of the bar code wand that is transmitted to the light detectors utilized.
In the past, bar code symbols were sometimes illuminated along more than one path. However, both this and the conventional single path approach suffer from a limited depth of field. This results in the bar code wand being useful only over a range. It has been discovered that placing a plurality of detection optical axes at an angle to the lens of the wand can increase the distance over which a bar code wand can be used.
The present invention provides an improved apparatus for reading bar symbols with separate illumination and detection optical axes.